Many different techniques have been proposed over the years for removing contaminants from hazardous waste sites, such as contaminated soil, all of which have suffered from one or more disadvantages which have made their use either technically or economically impractical. As used herein, soil can generally be understood to mean an earth-like medium having porosities ranging from a very densely packed clay medium to a relatively loosely packed medium, such as loosely packed sand.
The excavation and subsequent treatment of contaminated soil, for example by soil washing or incineration, is a costly technique and may expose workers using such process to health risks. Moreover, in the case of soil washing, the procedure may not extract all of the contaminants attached to the clay or silt components of the soil, while in the case of incineration, a site pollution problem may be replaced by the creation of an air pollution problem.
In situ collection and injection remediation techniques have also been proposed. Collection techniques, such as the collection of a contaminant plume by pumping and/or drains, often suffer from dilution by surrounding ground water during collection, thus increasing the pumping and treatment costs. Further, effective control of the direction of the flow is generally not possible due to soil heterogeneity and cracks.
Injection techniques, such as by using chemicals or biological agents injected in situ into the soil to detoxify the wastes, suffer from the difficulty of achieving a uniform distribution of the detoxifying materials throughout the soil. Moreover, both collection and injection techniques based on the use of pressure driven liquid flows may be impossible to use in soils having low hydraulic permeability so that their use is generally limited to relatively high permeability soils, e.g., relatively sandy soils. Also, when using presently proposed in situ remediation methods, including high pressure soil flushing, vacuum or steam extraction, or radio frequency volatilization, many contaminant materials, particularly heavy metals, cannot be removed because of the strong attachment forces which bind the metals to the soil particles.
Electroosmosis has been proposed for the dewatering and consolidation of clays or other soils to provide soil stabilization, such as for construction purposes, and for the purposes of removing contaminants from soil by transporting the contaminants with the flowing water. As used herein, electroosmosis is defined as the process of moving a liquid through a porous material by the application of an electric field. In accordance therewith voltage gradients are established in the soil and the water therein is thereby caused to migrate toward and accumulate at or near one of the electrodes which are used to create the electric field therein, the. accumulated water therein being removed therefrom, as by pumping.
Electromigration is a process that utilizes an electric field applied to the soil to transport contaminants by means of attracting ionically charged particles toward the electrodes with or without significant mass flow of fluid. Such a process is especially useful for removing metal contaminants from soil in-situ using electrodes to create an electric field. The contaminant ions may move in the same direction as the fluid flow or they may move in the opposite direction as the fluid flow in the electric field. A term that encompasses both the process of electroosmosis and electromigration is the term electrokinetics.
An electroosmosis system is described in U.S. Pat. No. 5,074,986 wherein at least one and, preferably, a plurality of porous anode electrode structures and at least one and, preferably, a plurality of porous cathode electrode structures are positioned at selected locations and at selected depths within a contaminated soil region. Such a system may also be applied to soil which has been removed from below ground and has been piled at a suitable location on the surface of the ground. The electrode structures are designed, for example, so that they are in the form of channel structures, such as tubular channels extending from the surface to below the contaminated region, the portions of the electrode structures below the surface within the contaminated region being porous, or perforated. One means of placing such tubular electrode structures in the soil would be to bore a hole in the soil and insert the electrode. In the case of a conventional well electrode, the hole would be bored, an electrode rod inserted and a porous fill, such as gravel or sand would be backfilled into the bored hole. Such boring, however, produces waste soil that must be separately decontaminated. Depending on the polarity of the charge of the soil, the electroosmotic flow can be either toward the anode electrode structures or the cathode electrode structures. In a positively charged soil, for example, electroosmotic flow will be toward the porous anode electrodes, whereas in a negatively charged soil the flow is toward the porous cathode electrodes. In the description below an electrode structure from which the flow emanates is called the "source electrode," and an electrode structure to which the flow migrates is called the "sink electrode."
A non-contaminating purging liquid, such as water, is supplied to the one or more source electrode structures so as to flow into the channel thereof and outwardly therefrom through the perforated portions thereof into the pores of the contaminated soil region. Voltage gradients are established between the source electrodes and sink electrodes by applying DC voltages thereto to create electric fields between source and sink electrodes. The contaminated liquid in the pores of the soil is displaced by, and accordingly, moved through the pores by, the non-contaminating purging liquid which purging liquid is itself moved through the pores as a result of electroosmosis. In some cases, depending on the nature of the contaminated liquid in the pores, and in particular with aqueous solutions, in addition to being moved by the purging liquid, such contaminated liquid may also be moved through the pores directly by electroosmosis.
The contaminant liquid moving through the pores flows into the one or more sink electrodes through the perforations therein and can then be removed to the surface through the sink electrode channel structures using suitable pumping or siphoning action, for example. The contaminant can thereupon be suitably collected at the surface.
By controlling the applied DC voltage levels, the number of electrode structures, and the depths and spacings of the electrode structures so as to control the directions and interaction of the voltage gradients produced between the electrode structures, the system can be operated in an effective manner, being particularly useful in waste sites having relatively low hydraulic permeabilities lying in a range of about 10.sup.-3 cm/sec. or less, comprising clays or the like.
There is a problem with the use of electrode wells or tubular electrodes that are distributed in a contaminated portion of soil in an equidistant array where the distance between like electrodes is about the same as, or sometimes slightly greater than, the distance between unlike electrodes. In this type of conventional array, it is believed that a highly non-uniform electrical field is produced because of unequal current paths between unlike electrodes. This is the so-called two-dimensional field effect versus the so-called one-dimensional field effect that has been observed in laboratory experiments with electroosmosis electrodes as reported in a paper entitled "Fundamental Aspects of Removing Hazardous Materials from Soils by Electric Fields". This paper was presented by Ronald F. Probstien at the July 1994 proceedings of the Electric Power Research Institute (EPRI) Workshop on In Situ Electrochemical Soil and Water Remediation. In the one-dimensional experiments, a small cylindrical soil sample having dilute aqueous phase organics is contained between two electrodes that form caps to the cylindrical container. In this arrangement a uniform field is produced in the soil between unlike electrodes. In this situation, after about 1.5 pore volumes of fluid is removed from the sink electrode, more than 90% of the contaminant is removed from the soil. One pore volume is the volume of liquid that can be contained in one volume of soil. In a companion two-dimensional experiment, the soil is contained in an open top rectangular box and tubular electrodes are placed near the ends of the box. In this arrangement, non-uniform convection velocities and removal rates are observed and more than 4.0 pore volumes of fluid are removed at the sink electrode before the same level of contaminant removal is achieved. The author concludes that ". . . the electroosmotic velocity distribution resulting from a particular electrode configuration determines the efficiency of the removal process, with high efficiency in the area between the electrodes, while the area outside the electrodes is not as effectively purged." In such a system using an equidistant row of electrodes, the soil must be treated for a time sufficient to remove the contaminant from the portion of soil having the longest current path. This results in great inefficiencies in electrical power consumed and in the extended time required to treat a volume of soil. It has been estimated that due to the non-uniformity of the electric field between tubular electrodes, the number of pore volumes of fluid forced through the soil sample to achieve a high decontamination level between 90-100% is about 2.times. to 4.times. that required where a uniform field is established. If the extended time to move more fluid is to be compensated for by a higher flow rate achieved by closer spacings of unlike electrodes and higher electrical currents at the same voltage, there is an economic problem that more electrodes are required and more electrical energy is required, and additionally there may also be a problem with heating of the soil at high currents that tends to boil off the water thereby creating nonconductive regions.
Another means of achieving an electrode in the ground is to dig a ditch in the ground to a suitable depth and then backfilling the ditch with electrically-conducting powder particles. Terminal electrode rods are then imbedded at suitable gaps in the powder particles to form an electrode wall in the ground. Opposing electrode walls are connected to an electrical voltage so that one of the walls acts as a cathode and one as an anode. The electroosmosis occurs between the opposite electrodes. In Japanese patent publication 5-336842 such a system is used to remove salts from soil. Such a system has a problem in that a large quantity of soil must be removed and then disposed of to provide a ditch, and the ditch depth may be limited before problems with collapse of the ditch walls during digging occurs. In cases where the soil is contaminated with a hazardous substance, handling the soil is a major concern.
Another means for cleaning contaminated ground water in soil is to pass the water through a permeable mixture of activated carbon and iron filings. The activated carbon acts to retard the contaminant while letting the water go through. The retained contaminant remains in contact with the iron filings for an extended residence time so chemical reaction with the iron filings can break down the contaminants into harmless or less hazardous substances. The permeable mixture is applied by digging a trench that extends below the water table in an aquifer in the path of a plume of the contaminant. The mixture would be backfilled into the trench. In British patent publication GB 2,255,087 A to Gilliam, such a system is used to clean halogenated organics, including chloroform, trichloroethane, solvents, pesticides, etc. from ground water. Such a system is only effective in a loosely packed soil medium that facilitates water flow by gravity induced pressure. Such a system has a problem in that a large quantity of soil must be removed and then disposed of to provide a ditch, and the ditch depth is limited to only a few feet before problems with collapse of the ditch walls during digging occurs. In cases where the soil is contaminated with a hazardous substance, handling the soil is a major concern. Further details of such a process are contained in U.S. Pat. No. 5,266,213 to the same inventor.
Another means to handle contaminants in soil is to emplace electrode materials and treatment materials in the soil and use electroosmosis to drive the contaminants through the treatment materials as described in U.S. Pat. No. 5,398,756 to Brodsky et al and U.S. Pat. No. 5,476,992 to Ho et al, which patents are hereby incorporated herein by reference. Such a system uses trenching or soil fracturing to place the materials in the soil which is time consuming and, in the case of trenching, requires separate disposal and treatment of a significant amount of excavated material.